TOI-1518b: A Misaligned Ultra-hot Jupiter with Iron in Its Atmosphere

Abstract

We present the discovery of TOI-1518b—an ultra-hot Jupiter orbiting a bright star ( V = 8.95). The transiting planet is confirmed using high-resolution optical transmission spectra from EXPRES. It is inflated, with R p = 1.875 ± 0.053 R J , and exhibits several interesting properties, including a misaligned orbit ( 240.34 − 0.98 + 0.93 degrees) and nearly grazing transit ( b = 0.9036 − 0.0053 + 0.0061 ). The planet orbits a fast-rotating F0 host star ( T eff ≃ 7300 K) in 1.9 days and experiences intense irradiation. Notably, the TESS data show a clear secondary eclipse with a depth of 364 ± 28 ppm and a significant phase-curve signal, from which we obtain a relative day–night planetary flux difference of roughly 320 ppm and a 5.2 σ detection of ellipsoidal distortion on the host star. Prompted by recent detections of atomic and ionized species in ultra-hot Jupiter atmospheres, we conduct an atmospheric cross-correlation analysis. We detect neutral iron (5.2 σ ), at K p = 157 − 44 + 68 km s −1 and V sys = − 16 − 4 + 2 , adding another object to the small sample of highly irradiated gas-giant planets with Fe detections in transmission. Detections so far favor particularly inflated gas giants with radii ≳1.78 R J , which may be due to observational bias. With an equilibrium temperature of T eq = 2492 ± 38 K and a measured dayside brightness temperature of 3237 ± 59 K (assuming zero geometric albedo), TOI-1518b is a promising candidate for future emission spectroscopy to probe for a thermal inversion.